1. Field of the Invention
The present invention relates to a switch box provided in a power supply system etc. of a vehicle, and an overcurrent preventing method.
2. Description of the Related Art
For example, a power supply system for a vehicle is provided with a main battery and a second battery which is a spare battery for backup. In the power supply system, a switch box including a switching element is provided between the main battery and the second battery, and supply of a current from the second battery is controlled by the switching element of the switch box (for example, see JP-A-2013-198318).
When an overcurrent flows into the switching element inside the switch box, there is a fear that the switching element may be damaged. Therefore, in the switch box, a shunt resistor is provided to monitor a voltage drop in the switching element to detect an overcurrent. Thus, the switching element can be turned OFF before the switching element is damaged. Although it may be considered that an overcurrent can be therefore prevented from flowing into the switching element, the provision of the shunt resistor leads to increase in size of the switch box. In addition, due to the shunt resistor serving as a heat generating component, there is a fear that temperature requirement cannot be satisfied. In addition, it may be also considered that a current sensor such as a current transformer for detecting a current flowing into the switching element is provided to monitor a current value detected by the current sensor to thereby prevent an overcurrent from flowing into the switching element. Also in this case, there is a problem that the size of the switch box may be increased due to the provision of the current sensor.